I'm putting together a little automotive project with a Uno board and sensor shield from yourselves, connected to a an I2C 20x4 display that I already had. I'm hoping to add a relay board for the Arduino to switch, but I'm conscious about the power capabilities of the board.

So, two questions really:

1) Will the 5v relay modules you sell be suitable for switching directly from the sensor shield / Uno board ?
2) Am I likely to start running into power issues with all these loads being powered from the Uno ?

1) Will the 5v relay modules you sell be suitable for switching directly from the sensor shield / Uno board ?

Yes, this is exactly what they are designed for. Depending on the relay module you choose they either have an transistor or opto-isolator interface that means there is very little load placed on the digital pin used to drive them.

2) Am I likely to start running into power issues with all these loads being powered from the Uno ?

Both the LCD and the relay modules require a 5V power supply source. If you're using your Arduino to power them I would try and keep total current draw from its 5V supply below 0.5A. If your powering the Arduino from the USB port you wont be able to draw more than this anyway.

For the 2004 LCD most of its power requirements come from the backlight so you will need about 40mA for this.

For the relay(s) you will need to allocate about 60 to 70mA per relay whilst they are energised. There will be a bit of in-rush current when the relay is first energised but if you avoid switching all the relays on at the same time you should be able to power a 4 channel relay module and the LCD from the Arduino's 5V supply without any issue.

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That's great thanks. I'm only likely to be using a single relay or two at the most, so I think I'll be fine. I've built a 9V 1A regulated supply to feed the Uno from the vehicle power, taking advantage of the on-board regulator to power the LCD and keep the wiring a bit simpler. Hopefully everything will work without trouble, but I can always add a 5V output to the supply for the external components if necessary. I was mostly concerned about damaging the Uno, but it sounds like it's going to be fine.